A vesicular system may be considered as a particular type of emulsion system, in which the dispersed or emulsified phase particles are layered vesicles which are suspended in the continuous phase.
The fact that vesicular systems can be formed and then used to entrap and carry desirable active compounds is well known. Such systems have most frequently been formed from organic materials of biological origin such as lipids (see, e.g., U.S. Pat. No. 4,772,471), Vitamin E (U.S. Pat. No. 4,861,580), or steroids (U.S. Pat. No. 4,917,951), and have been especially used in the pharmaceutical fields to provide carriers for biologically active materials.
A method of making, from non-phospho-lipid surfactants, paucilamellar vesicles having a central cavity substantially filled with a water-immiscible oily material is disclosed by U.S. Pat. No. 4,911,928 to Wallach, Paucilamellar Lipid Vesicles. The "lipid vesicles" disclosed by this patent are large (500 nm diameter minimum) multilayered liposome-like structures which are centrifuged (at 10,000-14,000 rpm for 15 minutes) out of the system after formation.
The creation of a vesicular dispersion from non-ionic surfactants is disclosed by U.S. Pat. No. 4,536,324 to Fujiwara et al., Nonionic Surfactant Type Vesicle Dispersion, which discloses a vesicle system formed from non-ionic surfactants such as polyoxethylene castor oil ethers or hardened castor oil ethers combined with sorbitan polyesters of long chain fatty acids in water. Conventional mixing means, from mechanical to ultrasonic, are used to form the vesicle dispersion or emulsion. Suggested uses for the dispersion or emulsion are either alone as a cosmetic cream or lotion or for containing a lipophilic or hydrophilic pharmaceutically active component.
The creation of a vesicle system from a mixture of cationic and anionic surfactants in water has been reported by Kaler et al. (Science, Sep. 22, 1989, p. 1371) Gentle mixing of cetyltrimethyl ammonium tosylate and sodium dodecyl benzene resulted in immediate and spontaneous (no mechanical agitation) generation of vesicles having particle sizes between 30 and 80 nm. The vesicles so formed were said to be stable and able to efficiently encapsulate glucose or other solutes.
Many two component systems for the delivery of an aerosol from a pressurized container are known. One component of such a system must be a gaseous propellant. The other component is a liquid component, which contains the active ingredient to be dispersed, may be comprised of various solvents, or may have an aqueous base with added solvents.
Many propellants currently used (since the use of nonflammable chlorinated fluorocarbons is being limited for environmental reasons) are flammable hydrocarbon gases such as propane, butane, and isobutane. Aqueous systems are preferred for use with such propellants, since such a system can limit or even obviate the flammability of the propellant phase.
The use of microemulsions in an aerosol system is known. U.S. Pat. No. 4,655,959, Preparation of Non-Flammable Aerosol Propellant Microemulsion System, and U.S. Pat. No. 4,536,323, Non-Flammable Propellant Microemulsion System, both issued to Stopper, disclose an aerosol system which, when shaken in its container, forms an oil-in-water microemulsion. This microemulsion structure allows the amount of propellant to be increased up to 50% by weight without flammability problems. Stopper's reason for wishing to have a higher level of propellant than the 15% to 25% conventional limit for non-flammability is his desire to be able to disperse the entire contents of the dispenser.
A countervailing consideration to the desire to incorporate a large amount of propellant into an aerosol system for efficiency of delivery is the desire to limit the amount of volatile organic compounds (VOCs) released into the earth's atmosphere. More immediately critical is the need to reduce the amount of VOCs into the home, for there is concern that indoor air pollution may sometimes exceed external air pollution and this is becoming an issue as a potentially health affecting condition. California, for example, is developing maximum VOC concentration regulations for different product categories. The proposed limit for air freshener double phase aerosols is 30%; for insect 15 repellents, it is 65%. Analysis of one of the lowest VOC-containing aerosols currently on the market shows a VOC content of 28%.
An article in Aerosol Age ("CARB/Industry Negotiate Consumer Product Regs.", Jul. 1990, pp. 22-27) has a table showing the differences between the proposed limits and "industry's needs". Industry need for air fresheners and disinfectants are said to be 70%; for dusting aids, 35%; for hair sprays, 80%. The present invention appears to offer a system that can deliver a variety of active ingredients, making it useable for a wide variety of products, with a total VOC content far below the "industry needs".
Which compounds qualify as VOCs will depend on type as well as molecular weight but, in general, organic compounds with fewer than nine carbon atoms are usually considered potential VOCs. Propane, butane, and isobutane are, obviously, VOCs.
Thus a non-flammable aqueous aerosol system that could effectively deliver the container contents with a lower, rather than a higher, level of propellant is highly desirable both for environmental and regulatory reasons. However, prior art has not only not produced such a system but, as in the Stopper patents, even teaches away from such a possibility.
All aerosol systems require a certain minimum propellant head space pressure to expel the contents of the container. Propellant head space pressure is dependent upon the interaction the propellant has with other substances in the container.
A container charged with propane alone (no other substances in the container) will exhibit a head space pressure of 100-110 psi (7031 gr/sq. cm-7734.1 gr/sq.cm). A container containing water that is then filled with propane will exhibit a head space pressure of 110-120 psi (7834.1 gr/sq. cm-8437.2 gr/sq. cm). When alcohol, glycerol, or a surfactant is added to the water, the head space pressure can be lowered. A mixture of 66% water, 30% ethanol, and 4% propane will exhibit a head space pressure of 50 psi, which is a near optimum head space pressure for an aerosol system which will produce a spray. 55 psi (3867 gr/sq. cm) pressure is considered the optimum figure.
A further consideration for an effective aerosol system is the ability of the system to maintain the desired pressure as the contents and the propellant are expelled. The alcohol-water-propane system described above exhibits progressively decreased head space pressure as the contents are expelled from an aerosol container with a vapor tap valve.
A desirable aerosol system should thus have the capacity to entrap or reservoir some of the propellant phase and progressively release the propellant as the contents of the aerosol container are expelled, thus maintaining a constant-equilibrium head space pressure over most of the usable life of the aerosol container.